Environmental Review Toolkit
Water, Wetlands, and Wildlife

Wildlife and Habitat

Restoration of Fish Habitat in Relocated Streams


A favorable habitat for fishes will have an adequate supply of unpolluted water the year around, with sufficient dissolved oxygen and a favorable range of water temperature. It will have resting areas where the current is slow, an adequate food supply, a clean spawning area, protective cover where the fish can hide from predators, and a balanced ecosystem with few competitors for food and space. Such a habitat is preferred by the salmonid fishes-salmon, trout, charr-the common inhabitants of cold water streams with coarse material bottoms.

Resting and Hiding Areas

Fish can swim rapidly or maintain their position in swift water for only a short time. They spend most of their time in pools and eddies where the water velocity is less than 1 ft/sec. (0.3 m/sec.), making short forays into the current to seize passing morsels of food. While resting, the fish require shelter where they can hide from predators. They can find this shelter in undercut banks and tree roots, around logs, fallen trees and rocks, or under dense brush which hangs out over the water near the surface. Pools where the water is deep enough to make the bottom invisible, or a combination of depth and surface turbulence may also supply hiding cover.

Trout need a certain amount of living space and will defend their territory against intruders. Because of this territorial behavior a large pool may be less desirable than several smaller pools of the same total area. However, when there are many rocks in a large pool more fish will live there amicably.

Food Producing Areas

Riffles are important food-producing areas for stream fish. Here, during low and medium flows the current is swift enough to keep the stream bed free of sand and silt, yet not so swift as to displace the larger bed materials. Water velocities in these riffles generally exceed 2.5 ft/sec (0.75 m/sec). The fast water provides an ample supply of food and oxygen for insect larvae and nymphs which attach themselves to the bed materials or find shelter in the rock crevices. These insects occasionally float down into the pools where they are eaten by fish. Trout may also go into the riffles for short periods in search of food.

Stream fish will swim only short distances from their resting places to feed. Food-producing areas therefore should be adjacent to pools and hiding places. The riffle-pool sequence common to coarse bedded streams is an ideal arrangement for a productive fishery.

Reproduction Areas

At the proper season, which varies with the species, adult fish move upstream into the colder tributaries in search of gravel bottom areas suitable for nesting. Such areas will generally have a downward flow of water through the gravel to aerate the newly-laid eggs, a condition often found at the downstream ends of pools. Spawning gravel must be relatively free of silt and about 1/2 to 2 inches (13 to 51 mm) in diameter. The female moves the gravel with her tail to make a nest in which the eggs are laid. The gravel must then remain stable so that the eggs have a free flow of water over them until the fry emerge 40 to 45 days later. Very small amounts of silt deposited on the eggs in the nest will drastically reduce their chances for survival.

In most cold water streams spawning occurs during low and medium flows. For adequate reproduction, it is essential that barriers to migration do not exist during the spawning run. Such barriers are created in nature by beaver dams, by channel braiding or by waterfalls. Man-made barriers include dams, long culverts with high velocities or short culverts with wide floors which spread the water thinly. A vertical drop at the end of a culvert will also stop migration.

An insufficient amount of spawning space for the fish population will reduce reproduction. Spawning females may destroy earlier nests while making their own. This problem can be serious where a stream supports a spawning run from a large lake or reservoir.

Streamside Vegetation

Streamside vegetation improves fish habitat by stabilizing the banks against erosion and by shading the water from the heat of the sun. Terrestrial insects dropping from overhanging bank vegetation may be an important source of food for fish in some seasons of the year, and vegetal detritus contributes to the flow of nutrients which sustain the invertebrate population of a stream.

Questions and feedback should be directed to Dan Buford (Daniel.Buford@dot.gov, 202-366-8168).

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